The development of novel antagonists of HIV-1 replication at the entry level, via blockade of gp120/CD4 interactions and/or structural changes, represents. Synthetic combinatorial libraries (SCLs) made up of thousands to millions of heterocyclic compounds, peptidomimetics or peptides will be used to develop such antagonists. The strength of SCL approaches relies on the rapid identification of highly active compounds from large pools of individual compounds. A first aim is to screen 10 separate SCLs that have different chemical characteristics (i.e., N- alkylated peptides, dipeptidomimetics, polyamines, indole-pyrido- imadazoles, N-alkyl amino cyclic ureas and thioureas and bicyclic guanidines) for their ability to inhibit fusogenic activity mediated by HIV-1 recombinant glycoprotein using two assay systems mimicking the T- cell line-tropic (T-tropic) and macrophage-tropic (M-tropic). Deconvulation of these SCLs will be carried out to identify compounds having either antagonist activity in the two assay systems or specific activity in a given tropic system. A second strategy will apply the SCL approaches to the identification of peptides having optimized antagonist activities and/or conformations that would mimic those found in HIV-1 glycoproteins. This approach will consist of: 1) selecting key residues in known antiviral peptide sequences (e.g., C4 derivatives, amphipathic alpha-helical attitudes) based on existing and/or proposed traditional SAR studies; 2) generating a biased SCL in which these key positions are randomized; and 3) identifying those analogs having greater inhibitory activity relative to the original peptide scaffold. When necessary, the SCL approach will be subsequently applied to further optimize the newly identified compounds to reach activities in the nanomolar range, and/or to increase their biological stability and bioavailability. The final series of studies are designed to evaluate the level of activity of the newly identified compounds in HIV-1 infectivity using different prototype M-tropic and T-tropic isolates in replication assays. Furthermore, studies toward the understanding of the mechanisms of action of these antagonists will be initiated.